A memory device is a device where information typically in the form of binary digits can be stored and retrieved. Such a device includes dynamic random access memory (DRAM), static random access memory (SRAM), and flash memory. Despite being slower, DRAMs are more commonly used than other memory types because they can be fabricated in high density to store a large amount of information. SRAMs are usually reserved for use in caches because they can operate at high speed. Unlike both DRAMs and SRAMs, which retain information as long as there is applied power, flash memory is a type of nonvolatile memory, which will keep information even if power is no longer applied. Flash memory is typically not used as main memory, however, because its block-oriented architecture prevents memory access in single-byte increments.
Another memory type has emerged that can be fabricated in high density, operated at high speed, and retain information even after power is no longer applied. This memory type is magnetic random access memory (MRAM). FIG. 1A is a block diagram showing a portion of an MRAM array 100 according to the prior art. The MRAM array 100 includes a number of memory cells, such as memory cells 1061,1 to 1063,4, which are arranged in a number of rows (word lines), 1041 to 1043, and a number of columns (bit lines), 1021 to 1024. Each of these memory cells 1061,1 to 1063,4 stores information magnetically instead of electronically as in DRAMs, SRAMs, and flash memory. As an example, to select the memory cell 1062,3 for reading and writing, a row current Irow is issued over the row 1042 and a column current (Icol) is issued over the column 1023.
FIG. 1B is a partial cross-sectional isometric view of the portion of the MRAM array 100 according to the prior art. Each memory cell is sandwiched between a portion of a row and a portion of a column. Rows and columns are formed from strips of conductive material.
Following the example above, when the row current Irow is present in the row 1042, the magnetic field Hy that is generated by this current partially selects memory cells 1062,1 to 1062,4. When the column current Icol is present in the row 1023, the magnetic field Hx that is generated by this current partially selects memory cells 1061,3 to 1063,3. Because memory cell 1062,3 is exposed to both magnetic fields (Hx and Hy), it is fully selected for reading or writing information.
FIG. 1C is an exploded isometric view of the memory cell 1062,3 and a portion of the row 1042 and the column 1023 according to the prior art. The row current Irow creates the magnetic field Hy that comprises a magnetic flux line 108 and the column current Icol creates the magnetic field Hx that comprises a magnetic flux line 110. These magnetic flux lines, 108 and 110, change the dipolar orientation of the memory cell (north or south) 1062,3. In this way, by taking advantage of the dipolar nature of a magnetic material that comprises the memory cell 1062,3, a bit of information can be represented as a 0 or a 1.
FIG. 1D is a graph showing the ferromagnetic nature of the memory cell 1062,3 according to the prior art. The graph shows a hysteresis loop 112, which shows the relationship of induction B as a function of magnetic field strength, H. With a sufficient coercive field x, applied to the memory cell 1062,3, the magnitude of the induction B rises until it levels off at a saturation induction, Bs0. The coercive field Hc is a combination of the magnetic fields Hx and Hy. As the coercive field Hc is removed by withdrawing power to the memory cell 1062,3, much of the induction B is retained by dropping its magnitude to a remanent induction Br0. This ability to retain the induction B even after power is no longer applied allows each memory cell of the MRAM array 100 to be nonvolatile. The induction B can be moved to another saturation induction, Bs1, by the application of the coercive field Hc. When power is again withdrawn, the magnitude of the induction B drops slightly to settle at a remanent reduction Br1. A bit of information can be magnetically represented as a 0 or a 1 by forcing the induction B to settle at the remanent induction Br0 or Br1.
FIG. 1E is a graph showing the ferromagnetic nature of the memory cell 1062,3 as a relationship between resistance R and coercive field Hc according to the prior art. This relationship is shown as a hysteresis loop 114, which illustrates that the memory cell 1062,3 exhibits a high resistance RH at one magnetized orientation (remanent induction Br0) and a low resistance RL at another magnetized orientation (remanent induction Br1). As a practical matter, it is less complicated to measure resistance to determine whether a 0 or a 1 is being stored by the memory cell 1062,3 than to measure the induction B as shown in FIG. 1D.
FIG. 1F is a graph showing the coercive field Hc that defines the relationship between the magnetic field Hy, which is formed from the row current Irow, and the magnetic field Hx, which is formed from the column current Icol according to the prior art. The shaded area 1160, which is underneath the curve of the coercive field Hc, defines a region where the memory cell 1062,3 is partially selected but is not sufficiently selected for reading and writing information despite the application of one or both the magnetic fields Hx and Hy. The area 1180, which is above the curve of the coercive field Hc, defines a region where the memory cell 1062,3 is fully selected because both the magnetic fields Hx and Hy are of a sufficient magnitude. The dashed line 120 illustrates an application of both the magnetic fields Hx and Hy at the same magnitude to select the memory cell 106, and to unselect (or partially select) memory cell 1062,3 when only one of the magnetic fields Hx and Hy is applied.
FIG. 1G is a graph showing a full-select probability distribution 1181, which represents a range of Hx where the memory cell is fully selected, and a partial-select probability distribution 1161, which represents another range of Hx where the memory cell is partially selected, according to one embodiment of the present invention. The probability distribution 1181 reflects the application of both the magnetic fields Hx and Hy at the same magnitude to fully select the memory cell 1062,3. The probability distribution 1161 reflects the application of only the magnetic field Hx but not Hy to unselect (or partially select) the memory cell 1062,3. As shown, a portion of the area under the probability distribution 1181 overlaps with a portion of the area under the probability distribution 1161. This overlapped area indicates that an ambiguity exists in the process of selecting the memory cell 1062,3. For example, in certain circumstances, the memory cell 1062,3 may be fully selected even though only the magnetic field Hx is applied. This accidental selection of a memory cell may compromise the integrity of the data stored by the memory cells.
Without a solution to unambiguously select a magnetic memory cell for reading and writing information, consumers may question the reliability of this type of memory device, which may lead to its eventual lack of acceptance in the marketplace. Thus, there is a need for structures and methods to increase the reliability of magnetic memory devices.